Fix undefined behaviour in float<->int conversion

Fix undefined behaviour in float<->int conversion

Summary:
This change fixes a case of undefined behaviour when converting between
floating point and integral values using folly::to<>. Undefined behaviour
is triggered when a floating point value is cast into an integral value
that cannot represent the source value. This happens in both cases,
float-to-int and int-to-float conversion, with folly::to<> due to the
check for loss of precision.

The new test cases expose the undefined behaviour.

The fix is a series of extra checks, the majority of which will only
kick in if the value to be converted is close to the boundary of the
range of the target type. These checks ensure that the conversion will
only be performed if the source value is within the range representable
by the target type.

The extra checks /will/ make the code slower. However, a later change
in this series, which refactors the implementation of folly::to<>, will
restore some of the performance.

Reviewed By: yfeldblum

Differential Revision: D3433757

fbshipit-source-id: 43495d18f831206ef48f74332663263d789a4f8a